Looking for a platform/site where I can render my scene at no cost
First - there is no free rendering.

With that out of the way here is one option - https://www.sheepit-renderfarm.com/

If you ask me, your best bet would be to optimize your own setup or if you are planning on seriously getting into Blender invest into a rendering setup. 
What are you trying to render and what is your setup(both hardware and settings in Blender).